mtd: rawnand: gpmi: cleanup makefile

The extra gpmi_nand.o object is not needed anymore since
commit 3045f8e3 ("mtd: rawnand: gpmi: move all driver
code into single file").

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201007134533.31390-1-m.felsch@pengutronix.de